Studies of sodar‐observed dot echo structures

Abstract

During moist weather under stably stratified and light wind conditions very often “dot” shaped echoes, either distributed randomly or arranged in a stratified layer, have been observed on sodar echograms. They last from a couple of hours to ten hours. Their horizontal widths are up to 200 m while their vertical sizes are up to 40 m. It is argued that dot echoes represent clusters of water vapour translated by the wind in the boundary layer, the back‐scattered acoustic energy being the contribution of correlated fluctuations in temperature and humidity (turbulent mixing) in the inertial subrange.     

Global plasmaspheric TEC and its relative contribution to GPS TEC

The plasmaspheric electron content is directly estimated from the global positioning system (GPS) data onboard JASON-1 Satellite for the first time. Similarly, the ground-based GPS total electron content (TEC) is estimated using about 1000 GPS receivers distributed around the globe. The relative contribution of the plasmaspheric electron content to the ground-based GPS TEC is then estimated globally using these two independent simultaneous measurements; namely ground-based GPS TEC and JASON-1 GPS TEC. Results presented here include data from 3 months of different solar cycle conditions (October 2003, May 2005, and December 2006). The global comparison between the two independent measurements was performed by dividing the data into three different regions; equatorial, mid- and high-latitude regions. This division is essential as the GPS raypaths traverse different distances through the plasmasphere at different latitudes. The raypath length through the plasmasphere decreases as latitude increases. The relative contribution of the plasmaspheric electron content exhibits a diurnal variation that depends on latitude with minimum contribution (10%) during daytime and maximum (up to 60%) at night. The contribution is also maximum at the equatorial region where the GPS raypath traverses a long distance through the plasmasphere compared to its length in mid- and high-latitude regions. Finally, the solar cycle variation of plasmaspheric contribution is also reported globally.     

Solar coronal heating by magnetic cancellation – III. Thermodynamics

We present two-dimensional numerical magnetohydrodynamics simulations of a coronal X-ray bright point (XBP) caused by a cancelling magnetic feature (CMF). Cancellation is driven by converging motions of two magnetic bipolar sources. These sources are initially disconnected from each other so that both, the CMF and the associated reconnection/heating event (i.e. the XBP), are modelled in a self-consistent way. In the initial state, there is no X-point but two separatrices are present. Hence, the reconnection/heating and the cancellation phases have not yet started. Our numerical experiments end shortly after the converging magnetic bipole has fully cancelled. By this time, reconnection in the inner domain has ceased and occurs only at the base. Solving the energy equation with various heating and cooling terms included, and considering different bottom boundary conditions, reveals that the unrealistically high temperatures produced by Ohmic heating are reduced to more moderate temperatures of 1.5–2 MK consistent with observations of XBPs, if thermal conduction is included and density and temperature are fixed at the base.     

Retrowedge-related Carboniferous units and coeval magmatism in the northwestern Neuquén province, Argentina

The studied Carboniferous units comprise metasedimentary (Guaraco Norte Formation), pyroclastic (Arroyo del Torreón Formation), and sedimentary (Huaraco Formation) rocks that crop out in the northwestern Neuquén province, Argentina. They form part of the basement of the Neuquén Basin and are mostly coeval with the Late Paleozoic accretionary prism complex of the Coastal Cordillera, south-central Chile. U–Pb SHRIMP dating of detrital zircon yielded a maximum depositional age of 374?Ma (Upper Devonian) for the Guaraco Norte Formation and 389?Ma for the Arroyo del Torreón Formation. Detrital magmatic zircon from the Guaraco Norte Formation are grouped into two main populations of Devonian and Ordovician (Famatinian) ages. In the Arroyo del Torreón Formation, zircon populations are also of Devonian and Ordovician (Famatinian), as well as of Late Neoproterozoic and Mesoproterozoic ages. In both units, there is a conspicuous population of Devonian magmatic zircon grains (from 406?±?4?Ma to 369?±?5?Ma), indicative of active magmatism at that time range. The ε_Hf values of this population range between ?2.84 and ?0.7, and the TDM-(Hf) are mostly Mesoproterozoic, suggesting that the primary sources of the Devonian magmatism contained small amounts of Mesoproterozoic recycled crustal components. The chemical composition of the Guaraco Norte Formation corresponds to recycled, mature polycyclic sediment of mature continental provenance, pointing to a passive margin with minor inputs from continental margin magmatic rocks. The chemical signature of the Huaraco Formation indicates that a magmatic arc was the main provenance for sediments of this unit, which is consistent with the occurrence of tuff—mostly in the Arroyo del Torreón Formation and very scarcely in the Huaraco Formation—with a volcanic-arc signature, jointly indicating the occurrence of a Carboniferous active arc magmatism during the deposition of the two units. The Guaraco Norte Formation is interpreted to represent passive margin deposits of mostly Lower Carboniferous age (younger than 374?Ma and older than 326?Ma) that precede the onset of the accretionary prism in Chile and extend into the earliest stage of the accretion, in a retrowedge position. The Arroyo del Torreón and Huaraco formations are considered to be retrowedge basin deposits to the early frontal accretionary prism (Eastern Series) of Chile. The presence of volcanism with arc signature in the units provides evidence of a Mississippian magmatic arc that can be correlated with limited exposures of the same age in the Frontal Cordillera (Argentina). The arc would have migrated to the West (Coastal Batholith) during Pennsylvanian–Permian times (coevally with the later basal accretionary prism/Western Series). The source of a conspicuous population of Devonian detrital zircon interpreted to be of magmatic origin in the studied units is discussed in various possible geotectonic scenarios, the preferred model being a magmatic arc developed in the Chilenia block, related to a west-dipping subduction beneath Chilenia before and shortly after its collision against Cuyania/Gondwana, at around 390?Ma and not linked to the independent, Devonian–Mississippian arc, developed to the south, in Patagonia.     

Baryon loading and the Weibel instability in gamma-ray bursts

The dynamics of two counter-streaming electron–positron–ion unmagnetized plasma shells with zero net charge is analysed in the context of magnetic field generation in gamma-ray burst internal shocks due to the Weibel instability. The effects of large thermal motion of plasma particles, arbitrary mixture of plasma species and space charge effects are taken into account. We show that, although thermal effects slow down the instability, baryon loading leads to a non-negligible growth rate even for large temperatures and different shell velocities, thus guaranteeing the robustness and the occurrence of the Weibel instability for a wide range of scenarios.     

The separation of the influence of nutrients and climate on the varve time-series of baldeggersee, Switzerland

The varve data-set from a freeze-core taken in the deepest part of Baldeggersee was subjected to different multivariate statistical analyses in order to estimate the amount of variance in the varve thickness measures explicable by past climate and by the trophic state of the lake. A comparison of two different time-periods (1902 to1992 versus 1920 to 1980) revealed that the lake restoration programme since 1982 has had a significant impact on the formation of the seasonal layers. Results of the partitioning of the variance in the varve thickness measures showed that about two thirds of the variance are unexplained by a climate and trophic state model and that trophic state explains 6%, whereas climate accounts for about 28% of the variance before the effect of lake restoration had a strong impact on the varves. Among the climate parameters the amount of annual precipitation is a strong predictor for explaining the thickness of both dark layer and total couplet thickness, whereas summer precipitation is important for the thickness of the light layer.     

Synthetic seismograms from generalized ray tracing1

A new method of numerical computation of elastic wavefields in regions containing caustics is tested. The method is an extension of the asymptotic ray theory (ART). The essential features of the method consist of the application of expressions which are well defined at caustics and expressed in terms of ray tracing combined with complex ray tracing in caustic shadows. The method and an outline of the underlying theory are briefly presented, followed by a comparison with finite differences on a test model involving a caustic cusp. The comparison reveals the unexpectedly high degree of accuracy of the new method.     

Geology of the Acropolis prospect,South Australia,constrained by high-precision CA-TIMS ages

Abstract

Acropolis is an Fe-oxide–copper–gold prospect ～20?km from Olympic Dam, South Australia, and marked by near-coincident gravity and magnetic anomalies. Prospective Fe-oxide–apatite?±?sulfide veins occur in Mesoproterozoic and Paleoproterozoic volcanic and granitoid host units beneath unmineralised sedimentary formations. We have produced a geological map and history of the prospect using data from 16 diamond drill holes, including LA-ICPMS and high-precision CA-TIMS ages. The oldest unit is megacrystic granite of the Donington Suite (ca 1850?Ma). A non-conformity spanning ca 250 My separates the Donington Suite and felsic lavas and ignimbrites of the Gawler Range Volcanics (GRV; 1594.03?±?0.68?Ma). The GRV were intruded by granite of the Hiltaba Suite (1594.88?±?0.50?Ma) and felsic dykes (1593.88?±?0.56?Ma; same age as the Roxby Downs Granite at Olympic Dam). The felsic dykes are weakly altered and lack Fe-oxide–apatite–sulfide veins, suggesting that they post-date the main hydrothermal event. If correct, this relationship implies that the main hydrothermal event at Acropolis was ca 1594?Ma and pre-dated the main hydrothermal event at Olympic Dam. The GRV at Acropolis are the same age as the GRV at Olympic Dam and ca 3–7 My older than the GRV exposed in the Gawler Ranges. The gravity and magnetic anomalies coincide with sections through the GRV, Hiltaba Suite and Donington Suite that contain abundant, wide, Fe-oxide veins. The GRV, Hiltaba Suite and Donington Suite are unconformably overlain by the Mesoproterozoic Pandurra Formation or Neoproterozoic Stuart Shelf sedimentary formations. The Pandurra Formation shows marked lateral variations in thickness related to paleotopography on the underlying units and post-Pandurra Formation pre-Neoproterozoic faults. The Stuart Shelf sedimentary formations have uniform thicknesses.

1. KEY POINTS

2. Fe-oxide–apatite?±?sulfide veins are hosted by the Gawler Range Volcanics (1594.03?±?0.68?Ma), the Hiltaba Suite granite (1594.88?±?0.50?Ma) and Donington Suite granite (ca 1850?Ma).

3. The age of felsic dykes (1593.88?±?0.56?Ma) interpreted to be post-mineralisation implies that the main hydrothermal event at Acropolis was ca 1594?Ma.

4. The Gawler Range Volcanics at Acropolis are the same age as the Gawler Range Volcanics at Olympic Dam and ca 3 to 7 My older than the Gawler Range Volcanics exposed in the Gawler Ranges.

     

RECOGNISING THE UNCERTAINTY IN THE QUANTIFICATION OF THE EFFECTS OF CLIMATE CHANGE ON HYDROLOGICAL RESPONSE

A widely used method of evaluating effects of climate change on flow regime is to perturb the climate inputs to a rainfall–runoff model and examine the effect on a statistic of the modelled flows. Such studies require four elements: a method of perturbing the climate, a rainfall–runoff model, a study catchment and a flow index. In practice the direction and magnitude of the estimated effects depend on each of the four elements, leading to concern over the usefulness and generality of the results. To investigate these uncertainties two climate scenarios and eight climate sensitivity tests have been applied to three UK catchments using two conceptual rainfall–runoff models in order to quantify effects of climate change on three flow indices representing mean runoff, flood magnitudes and low flows. The sensitivity tests were found to be useful to assess the suitability of the models to simulate flows outside the conditions experienced in their calibration. Both models gave internally consistent results but, on close examination, one model was found inappropriate for this application. Results show that the effect of climate change on flow varies between catchments and that different flow response indices can change in opposite directions, e.g. floods increased in magnitude while low flows reduced. Contrasting results were obtained from the two climate scenarios.